Lazarus-main.zip is typically a source code archive downloaded directly from the development repository (often GitLab or GitHub). It contains the "main" (development) branch of the software, which is the most current, albeit sometimes less stable, version of the environment. Quick Setup Guide
Open a terminal/command prompt in the extracted folder and run: make clean all : Lazarus-main.zip
Once the build completes, look for the lazarus.exe (Windows) or lazarus (Linux/macOS) executable in the root folder. Why use "main.zip" instead of an installer? Lazarus-main